No you need to manually split the vertices if you want to allow this. Smoothing groups are simply a way to get multiple vertex normals into a single vertex. It doesn't actually duplicate vertices in the exported file. Only in the GPU for rendering.
Looks like the vertices gets welded together either upon importing into Unity, or when you export the fbx. So it can't have the sharp transitions anymore.
What I meant is to have separated smoothing groups for the different colors, so the vertices splits along the color changes, so he can have the hard transitions.
It's not smoothing groups. Unity only supports 1 vertex color per vertex. So, you cannot color per polygon (which causes hard edges in vertex color, because edge vertices are then storing multiple colors). If you want hard edges in VC, either chamfer the edges with tight bevels (not recommended, long thin triangles are bad…